1. Field of the Invention
The present invention relates to a semiconductor apparatus and a method of detecting characteristic degradation of a semiconductor apparatus and, particularly, to a semiconductor apparatus and a method of detecting characteristic degradation of a semiconductor apparatus that detect characteristic degradation of a semiconductor apparatus which is a so-called intelligent power device (IPD).
2. Description of Related Art
A power MOSFET is one of devices capable of handling a large power. The power MOSFET has a feature such as a higher switching speed than other power devices.
Japanese Unexamined Patent Application Publication No. 2007-174756 discloses an on-failure detection apparatus of a power supply circuit that can protect the power supply circuit by detecting a sign of an on-failure of a semiconductor device (power MOSFET) which is used as a switching device that switches between on and off of the power supply circuit and turning off the semiconductor device at the point of time before a breaking function of the circuit is disabled. According to the technique disclosed in Japanese Unexamined Patent Application Publication No. 2007-174756, an on-failure caused by a dielectric breakdown can be detected in advance by placing a gate resistor at the gate of the power MOSFET and measuring a voltage drop of the gate resistor when the power MOSFET is on.
Further, Japanese Utility Model No. 2599788 discloses a technique related to a failure detector that can perform failure identification even when a power MOSFET is in a semi-failure state. According to the technique disclosed in Japanese Utility Model No. 2599788, in a switching module using a power MOSFET, two comparators that compare output voltages are placed separately, a means of determining that the power MOSFET is normally off and a means of determining that the power MOSFET is normally on are placed independently, and a failure identification circuit that receives the respective determination signals and identifies a failure is placed.
Further, “MOS Integrated Circuit (μPD166005) data sheet (NEC Electronics)” <URL:http://www.eu.necel.com_pdf/S19284EJ1V0DS00.PDF> discloses an intelligent power device (IPD). FIG. 25 is a view showing an exemplary circuit that incorporates the IPD disclosed in the document. An IPD 101 is placed between a VCC terminal 103 and a GND (ground) terminal, receives a signal output from an output terminal 108 of a controller 102 at an input terminal 106, and controls a load 109 which is connected to an output terminal 104. Further, the IPD 101 has a self-diagnosis function, and outputs a result of self-diagnosis to an input terminal 107 of the controller 102 through a DIAG terminal 105.
FIG. 26 is a detailed circuit diagram of the IPD 101 disclosed in the above “MOS Integrated Circuit (μPD166005) data sheet (NEC Electronics)”. The IPD 101 processes a signal received at the input terminal 106 by a logic 113 and controls a power MOSFET 110. In the power MOSFET 110, a drain is connected to a power supply terminal (VCC) 103, and a source is connected to the output terminal 104. A current that flows to the load 109 which is connected to the output terminal 104 is controlled by the power MOSFET 110. Further, the IPD 101 has a self-diagnosis function such as an over-current detector 111, an over-temperature sensor 112 or the like, and upon reaching a preset criterion, it can cut off the power MOSFET 110 and feed back the information as a self-diagnosis result to the controller 102 through the DIAG terminal 105. An operation when the over-current detector 111 and the over-temperature sensor 112 work is described hereinbelow.
First, a case where the over-current detector 111 operates in a load short circuit condition is described. FIG. 27A is a view showing a change in the output current of a power MOSFET when the power MOSFET turns on and then a load short circuit occurs. When a load short circuit occurs, an over-current exceeding the current rating flows to the power MOSFET, and the power MOSFET is broken by heat. Generally, a current limiter that avoids an over-current from flowing is placed in the IPD. By using the current limiter, an over-current can be suppressed to some level as shown in FIG. 27B. However, the heat increases with time in this case also, which leads to a breakdown of the power MOSFET.
To avoid this, the IPD shown in FIG. 26 includes the over-current detector 111. The over-current detector 111 has a function of cutting off the power MOSFET when a current flowing through the power MOSFET exceeds a current detection threshold which is preset to the IPD as shown in FIG. 27C. The function prevents a breakdown of the power MOSFET due to heat.
FIG. 28 is a view showing an example of the over-current detector 111. The over-current detector 111 includes a comparator 116, and a potential of the output terminal 104 is supplied to one input of the comparator 116, a current detection threshold 115 is supplied to the other input, and an output of the comparator 116 is input to the logic 113. When the on-resistance of the power MOSFET 110 is 100 mΩ and the current rating is 2 A, the current detection threshold is designed to 0.5V (=100 mΩ×5 A), for example. In such conditions, when the output current of the power MOSFET exceeds 5 A, a driver 114 is controlled by a logic 113, so that the power MOSFET 110 is cut off.
Next, a case where the over-temperature sensor 112 operates in a load short circuit condition is described. FIG. 29A is a view showing a change in the output current of a power MOSFET when the power MOSFET turns on and then a load short circuit occurs, and a temperature change of the power MOSFET. When a load short circuit occurs, a current exceeding the current rating flows to the power MOSFET, and the power MOSFET is broken by heat. Generally, a current limiter that avoids an over-current from flowing is placed in the IPD. By using the current limiter, an over-current can be suppressed to some level as shown in FIG. 29B. However, the heat increases with time in this case also, which leads to a breakdown of the power MOSFET.
To avoid this, the IPD shown in FIG. 26 includes the over-temperature sensor 112. The over-temperature sensor 112 has a function of cutting off the power MOSFET when the temperature of the power MOSFET exceeds a temperature detection threshold which is preset to the IPD as shown in FIG. 29C. The function prevents a breakdown of the power MOSFET due to heat.
FIG. 30 is a view showing an example of the over-temperature sensor 112. The over-temperature sensor 112 includes a comparator 118, and a voltage drop of a diode 119 which is thermally coupled to the power MOSFET is supplied to one input of the comparator 118, a temperature detection threshold 117 is supplied to the other input, and an output of the comparator 118 is input to the logic 113. A cathode of the diode 119 is connected to a constant current source 120. When the voltage drop of the diode 119 at 27° C. is 0.7 V, a temperature change is −2 mV/° C., and the temperature rating is 150° C., the temperature detection threshold is designed to 0.404 V (=0.7V-2 mV/° C.×(175° C.-27° C.)). In such conditions, when the temperature of the power MOSFET exceeds 175° C., a driver 114 is controlled by a logic 113, so that the power MOSFET 110 is cut off.
In this manner, in the IPD shown in FIG. 26, the over-current detector 111 and the over-temperature sensor 112 operate independent of each other, and a breakdown of the power MOSFET can be avoided under abnormal conditions.